1. Field of the Invention
The present invention relates to a fluorescent lamp and a liquid crystal display device having the same, and more particularly to a fluorescent lamp for a backlight of a liquid crystal display device capable of improved color reproduction and a liquid crystal display device having the same.
2. Description of the Related Art
Recently, to the rapid technical progress in the semiconductor industry, enabled electronic products to achieve improved performance, in a smaller size and with a lighter weight. A cathode ray tube (CRT), which is being widely used as an information display device, has a lot of advantages in view of performance and price-wise. However, its size cannot be minimized enough for portable uses. On the contrary, due to the advantages such as small sizen, lightweight and low power consumption, a liquid crystal display device is being noted as a substitute device to overcome the disadvantages of the CRT and now widely used for almost all of the information processing apparatuses which need display device.
The liquid crystal display device is a device that uses light modulation of the liquid crystal cells which applies a voltage to a molecular arrangement of liquid crystal to convert its molecular arrangement and changes the optical properties of liquid crystal cells that transmit light, such as a double refractivity, a polarization, a dichromaticism, and a light-scattering characteristic, as a visual properties.
Since the liquid crystal display device is a passive light element which is unable to emit the light by itself, a backlight assembly for providing the liquid crystal panel with the light is attached to the rear surface of the liquid crystal panel. A light transmissitivity of the liquid crystal panel is controlled by the applied electrical signal and, in response to the control, an image or a moving image is displayed on the liquid crystal panel.
As aforementioned above, since the liquid crystal display device controls the quantity of light transmitted onto the screen using the liquid crystal and decides a contrast and a color of the screen utilizing the light, the liquid crystal display device shows some different characteristics other than general display devices. For example, there are issues of an angle of visibility that makes the quality of the image remarkably different according to the angle of watching the screen, a transmissitivity according to a projection type light-emitting display, a color reproductivity according to how much the light passed through a color filter reproduces red R, green G and blue B colors, a luminance representing the contrast of the screen, and an after-image that remains for a long time after a same image stays long.
Currently, the liquid crystal display device, which has been mainly used in a display device of portable products, is now expanding its area into a desktop PC monitor and a television set. The liquid crystal display device has some physical advantages such as light-weight, thin-thickness, short-length and small-size, however, among the aforementioned characteristics, it is especially weak in view of the color reproductivity and a luminance in comparison with the CRT. Though the pre-existing liquid crystal display device for a notebook computer monitor has only about 40 to 50% of the color reproductivity compared to the national television system committee (hereinafter, referred to as “NTSC”) method which is adopted as a color television broadcasting method by American NTSC, it is still able to fulfill the needs of users. However, for a TV set noted as a new market for liquid crystal display device, of the liquid crystal display device needs to be developed to have at least same color reproductivity as the CRT or better color reproductivity is required.
Largely, a general multi-color liquid crystal display device comprises of the liquid crystal panel, the backlight and the color filter. That is to say, the multi-color liquid crystal display device utilizes a backlight of a three-wave fluorescent lamp as a light source and separates a white color light emitted therefrom through the color filter into three primary colors of red, green and blue, and then produces various colors by an additive color mixing method.
A color of an optional light source is decided by a chromaticity coordinates which is made by a Commission Internationale d'Eclairage (hereinafter, referred to as “CIE”). That is, after the tristimulus values X, Y and Z are calculated from the spectrum of the optional light source, the chromaticity coordinates of the red, green and blue x, y and z are obtained by a conversion matrix from the tristimulus values. Next, when the x, y and z value of the red, green and blue are represented as a vertically intersect coordinates, a horse's hoofs-shaped spectral locus is drawn. It is called as a CIE chromaticity diagram and all of the normal light sources have their chromaticity diagram inside the horse's hoof-shape. At this time, a triangle area made by each chromaticity coordinates of the red, green and blue becomes the color reproduction area and the larger the triangle area becomes, the more the color reproductivity increases. The color reproductivity depends on a saturation and a brightness and the color reproductivity increases when the saturation and brightness becomes higher. In here, the tristimulus values x, y and z represent the weighted values of an individual color-matching function approached to a certain spectrum, especially, y represents the stimulus values of the brightness.
Meanwhile, a color temperature indicates a color of the white color on the basis of color change of the light emitted according to the temperature of the heating source as the temperature, and the color temperature on the monitor is largely presented in three degrees, as 9300K, 6500K and 5000K. When the color temperature is near 9000K, the white color having the blue color is represented, when the color temperature is near 6500K, the white color having the red color is represented, and when the color temperature is near 5000K, an intermediate color is represented. The color temperature is obtained from the chromaticity coordinates x and y of the white color, and when the color temperature is near 9000K, it can satisfy the standard of European Broadcasting Union (hereinafter, referred to as “EBU”).
In case of the above-mentioned liquid crystal display device, the tristimulus values with respect to the each wavelength of a visible ray area is decided by compounding a luminous spectrum of the backlight with the color-matching function and a transmit spectrum of the color filter, so the co-relative relation between the backlight, the color filter and the tristimulus values should be adequately controlled to reproduce various color. Namely, the luminous spectrum of the backlight has to be changed to optimize the color reproductivity and the color temperature, and the transmit spectrum of the color filter should be adjusted to optimize a luminous efficiency.
Presently, the characteristics of the color filter formed as a color resist of a pigment dispersing type is as follows.
FIG. 1 is a graph illustrating a transmission spectral distribution of a high saturation color filter. In the graph, the “x” axis represents a wavelength (nm) and the “y” axis represents a transmission degree (%).
Referring to FIG. 1, the color reproductivity of the high saturation color filter is 67% on the basis of a standard white color light (C) when compared with to the NTSC method, a white brightness (Yw) is 30.5, and a color temperature is 6643K (chromaticity coordinates of the white color x=0.31, y=0.33). In comparison with the EBU standard, the green color and blue color are much insufficient. Here, the C light source means a standard daylight of a cloudy day, in another words, about 6774K degrees of the color temperature according to International Practical Temperature Scale.
In case of a recently developed color filter for the TV that meets the EBU standard, a method to control the matching ratio of each of the toning pigment is used to compensate the chromaticity coordinate values of the insufficient green color and the blue color. However, according to the method, a loss of the blue color transmission degree on the spectrum increases a lot in comparison with the pre-existing color filter. In developing the color filter for the TV, obtaining excellent color reproductivity at the expense of transmission degree is not a good approach because it wastes light source. Accordingly, it is designed to compensate the loss of the transmission by increasing the luminance of the backlight.
Presently, a cold-cathode ray fluorescent lamp is used as a light source for a the backlight of the liquid crystal display device. In light-emitting material and principle, the cold-cathode ray fluorescent lamp is able to be operated in low current, emits less heat and has longer lifetime without big differences from a general fluorescent lamp. The cold-cathode ray fluorescent lamp includes a glass tube with phosphor spread onto an inner wall and electrodes attached at both ends of the glass tube. The glass tube is sealed by a rare gas such as an argon (Ar) and a fixed quantity of hydrargyrum (Hg).
When a voltage is applied to the electrodes, electrons are emitted to ionize the gas in the glass tube. By the ionization and reunion of the ion and electron, a discharging of about 253.7 nm begins. The discharging excites the hydrargyrum (Hg) to generate a ultraviolet ray of 254 nm. The ultraviolet ray stimulates the phosphor spread onto the tube wall to emit a visible ray.
The fluorescent lamp developed earlier is not designed to consider the color reproductivity of the liquid crystal display device. Therefore, the concept of color separation of the three-wave light from the light source through the color filter is not included. Instead, development is progressed only in view of the color temperature, the high luminance, the long lifetime and the high efficiency. Accordingly, in the liquid crystal display device which needs various color realization of the light emitted from the three-wave phosphor having maximum luminous efficiency in each of the red, green and blue color area of the light source by means of the additive color mixing through the color filter, the cold-cathode ray fluorescent lamp presently used does not have a satisfactory luminous spectral shape.
Recently, in developing of a TV-liquid crystal display device with the high saturation, the color reproductivity is maximum 70% when applying a color filter for a television set. So, the TV-liquid crystal display device is under development in direction to move the place of the CIE chromaticity coordinates into the EBU area by means of adjusting the phosphor ratio of the fluorescent lamp to increase the color temperature of the white color. However, the gain range of the color reproductivity that can be obtained by adjusting the three-wave phosphor ratio is very small within about 2% maximum. Accordingly, the phosphor now in use has to be changed to or substituted with an appropriate phosphor for the TV-liquid crystal display device to obtain higher color reproductivity.